In order to ensure that paints coated on metal surfaces will exhibit their two principal functions, i.e., protection and decoration, for a prolonged period of time, it is often required to provide paint films in at least two layers. Heretofore, to meet this need, resins having improved adhesion and corrosion resistance have been used in primers, and resins having improved weatherability, stain resistance and hardness have been used in topcoats.
As for application techniques, there has been a shift to the two-coat, one-bake process from the two-coat, two-bake process. From the viewpoint of energy saving, the one-coat, one-bake process should ultimately be an ideal method.
Because of their high performance and low cost, demands for powder coatings have constantly been growing. On the other hand, powder coatings are applied in single layers which perform single functions, so there inevitably is a limit to the future growth of demands for powder coatings although their use is growing in applications that suit the specific performance of individual types of coatings.
Under these circumstances, active efforts are being made to study a resin composition of the polyesteracrylic hybrid type for powder coatings that possess both the features of a powder coating resin composition made of a polyester resin and the features of a powder coating resin composition made of an acrylic resin (JP-B-55-1945) (the term "JP-B" as used herein refers to an "examined Japanese patent publication"). With a view to improving the stain resistance and solvent resistance of blocked isocyanate cured polyester powder coatings, a composition characterized by a blend with an acrylic resin having at least one of a hydroxyl group and a glycidyl group has been proposed (JP-A-59-6267) (the term "JP-A" as used herein refers to a "published unexamined Japanese patent application").
Most of the conventional resin compositions of the polyester-acrylic hybrid type for powder coatings are characterized by merely blending polyester and acrylic resins. When polyester and acrylic resins that are not highly miscible with each other are blended, smooth coating surfaces are not attainable. On the other hand, polyester and acrylic resins that are highly miscible with each other are only capable of forming paint films that display a performance intermediate between those of the respective resins even in the most favorable case.